Scientists observed a planet the size of Earth zooming through the Milky Way, untethered to any star. An international team of scientists led by Polish astronomers discovered the smallest Earth-sized rogue planet found thus far. They believe our galaxy may have an innumerable amount of these free-floating planets. In fact, scientists have already discovered over 4,000 of them.
The terms extrasolar or exoplanets refer to any planet outside the solar system and orbits a star other than the Sun. The planet may travel through the galaxy freely without orbiting a star, like the one recently found. While most of the rogue planets have been observed to orbit a star, there are exceptions.
A few years ago, Polish astronomers from the Astronomical Observatory of the University of Warsaw discovered one of these planets. Publishing their observations in Astrophysical Journal Letters, they announced the existence of the smallest free-floating planet ever discovered in the Milky Way. Detecting these low mass objects in space proves difficult for scientists in most cases due to their distance from Earth. The lead study author says this means they either got very lucky or that countless other exoplanets exist.
How exoplanets are discovered
In most cases, astronomers discover planets by observing light cast from the stars they orbit. If an exoplanet passes in front of the star, the brightness decreases slightly. Called the transit method, this remains the most widely accepted way of detecting rogue planets. However, because free-floating planets don’t emit much radiation and don’t orbit stars, scientists can’t employ the traditional detection methods.
These planets use an astronomical phenomenon called gravitational microlensing, resulting from Einstein’s theory of general relativity. This says that a massive object, such as a star or planet, could bend the light emanating from a background object (the source). In other words, the objects’ gravity acts like a giant magnifying glass, which makes distant stars appear much brighter. This phenomenon can detect planets up to thousands of light-years away.
“If a massive object (a star or exoplanet) passes between an Earth-based observer and a distant source star, its gravity may deflect and focus light from the source. The observer will measure a short, brightening of the source star,” explains Dr. Przemek Mroz, a postdoctoral scholar at the California Institute of Technology and a lead author of the study. “Chances of observing microlensing are extremely slim because three objects—source, lens, and observer—must be nearly perfectly aligned. If we observed only one source star, we would have to wait almost a million years to see the source being microlensed,” he adds.
This explains why most gravitational microlensing takes place in the Milky Way’s center, where this method proves most successful. The OGLE survey, which Warsaw University astronomers oversee, uses this technique to discover rogue planets. OGLE, or Optical Gravitational Lensing Experiment, began over 28 years ago, finding at least 17 exoplanets since 1992.
Currently, OGLE astronomers use a 1.3-meter Warsaw Telescope at Las Campanas Observatory in Chile to look for microlensing signs. On clear nights, they scan the central regions of the Milky Way through the telescope, specifically searching for stars whose brightness fluctuates. Since this method doesn’t depend on the planets’ brightness, it can detect many objects that may otherwise be overlooked.
The duration of microlensing events depends on the mass of the planet being observed. The heavier the object, the longer the event will last, and vice versa for smaller planets or stars. Most microlensing events last for several days and result from stars. In comparison, events that observe rogue planets may last only a few hours due to a smaller mass. Scientists measure the length of the event and the shape of its light curve to estimate the mass of the object.
The shortest microlensing event ever recorded, called OGLE-2016-BLG-1928, lasted just 42 minutes. Models of the event show that the lens, or object being observed, probably equated to Mars’s mass. Scientists believe that they captured a rogue planet during this event.
What the scientists say
“When we first spotted this event, it was clear that it must have been caused by an extremely tiny object,” says Dr. Radoslaw Poleski from the Astronomical Observatory of the University of Warsaw, a co-author of the study. “If the lens were orbiting a star, we would detect its presence in the light curve of the event. We can rule out the planet having a star within about 8 astronomical units (the astronomical unit is the distance between the Earth and the sun).”
A few years ago, OGLE astronomers provided the first evidence of large numbers of rogue planets roaming in the Milky Way. The most recently discovered planet is the smallest free-floating planet ever found.